TH2.2 a mouse B cell hybridoma is induced to terminal differentiation by LPS. The terminally differentiated clones secrete IgM in inverse proportion to the number of cells in the clone. Terminal differentiation and sevretion of immunoglobulin were partly reversed by incubation induced cells in conditioned medium from uninduced cells. The terminally differentiated cells also produced a lymphokine with activity of GM-CSF. In terminal clones of intermediate size there was a strong correlation between [IgM]/1000 cells and [CSF]/1000 cells; e.g. The LPS was inducing 2 different activities in the same clones. F.16.47 is a B cell hybrid which responds to small doses of BSF-1 with a rapid (12 hours) and large (10 fold) increase in mRNA for Ia polypeptides, followed by an increase expression in Ia antigens on the membrane. Molecularly cloned BSF-1 produced this effect.